1. Field of the Invention
The present invention relates to a medium- to large-current power control device such as thyristors, TRIACs and the like.
2. Description of the Related Art
The power control devices, such as thyristors, TRIACs and the like, are preferably utilized as switches for hot-water supply systems. For example, a thyristor is a semiconductor device having three terminals of anode, cathode and gate. The semiconductor device operates as a switching element adapted to allow or prohibit a flow of current between the cathode and anode based on the current flow through the gate.
FIG. 14A is a front view for illustrating a typical power control device 1 whereas FIG. 14B is a side view thereof. A semiconductor component 3 is secured to a metal base 2 at the center thereof, and connected to terminals 4a and 4b. The semiconductor component 3 has its bottom surface bonded to the base 2 by soldering and its top surface connected to the terminals 4a and 4b via a wire 5. These are covered by a package formed of a synthetic resin material so as to form the power control device 1.
FIGS. 15 and 16 illustrate a first example of the prior art. FIG. 15 is a circuit diagram showing a circuit 11 employing the power control device 1. FIG. 16A is a plan view showing the semiconductor component 3 constituting the power control device 1 whereas FIG. 16B is a sectional view thereof taken on line 16B--16B in FIG. 16A. FIG. 16 illustrates an electrode configuration of the semiconductor component 3, in particular. The semiconductor component 3 of this configuration constitutes the power control device 1. The circuit 11 employing this power control device 1 is shown in FIG. 15. The circuit 11 includes a SSR (Solid State Relay) 12 comprised of a combination of multiple types of power control devices such as thyristors, TRIACs and the like. The circuit 11 is adapted to control heat generated by a load 13 by means of the SSR 12.
In some cases, an abrupt upsurge in current, such as caused by rush current, lightning surge or the like, may occur in the circuit including the power control device 1. Such a current is generally referred to as the surge current. A flow of the surge current may cause forward secondary breakdown which destroys a p-n junction of the semiconductor component 3 in the power control device 1. The power control device 1 guarantees to withstand the surge current of about 10 to 40 times the normal rated current by employing an electrode for semiconductor component 3, a semiconductor component 3 and a wire 5 for connection of terminals 4a and 4b, which are fully capable of withstanding the surge current.
As seen in FIG. 15, the circuit 11 incorporates a fuse 14 as an external element of the SSR 12. There is selected a fuse 14 such as to be fused when a fusing current of above a predetermined current value flows therethrough. The predetermined current value is greater than a withstand surge current of the power control device 1 constituting the SSR 12 and smaller than a current value at which the semiconductor component 3 in the power control device 1 is destroyed. When a current of above the withstand surge current flows through the circuit 11, the fuse is burnt even after the semiconductor component 3 is destroyed, so that the current flow through the circuit 11 is interrupted. This prevents the load 13 from generating excessive heat for assurance of safety.
Next, description will be given on the electrode configuration of the semiconductor component 3 constituting the power control device 1. As seen in FIGS. 16A and 16B, the top surface of a semiconductor chip 22 is masked by a masking oxide film 23 formed with contact windows for allowing electrodes to contact the semiconductor chip 22. The respective windows are filled with aluminum so as to define a cathode 24 and a gate 25. The cathode 24 and the gate 25 contact the semiconductor ship 22 at their respective portions occupying the windows. The bottom surface of the semiconductor chip 22 is covered by an anode 26 formed of a solderable metal (e.g., Ti--Ni alloy or the like). In the semiconductor component 3 of this configuration, the cathode 24 and gate 25 contact the semiconductor chip 22 at their respective bottom surfaces and have their respective top surfaces subjected to wire bonding.
In the power control device 1 comprising the semiconductor component 3 as shown in FIG. 16 is provided the cathode electrode 24 having a sufficient thickness and width in consideration of the maximum ratings and reliability of the device. This provides for the prevention of voltage drop caused by the cathode 24 serving as an interconnection resistance or fusions caused by the surge current and electro-migration. Particularly, there is defined a maximum possible contact area on which the cathode 24 contacts the semiconductor chip 22. Additionally, the wire 5 has a sufficiently large sectional area for withstanding the surge current.
According to a second example of the prior art (not shown), the fuse is replaced by a wire interconnecting the semiconductor component and external terminals, which also serves as a fusion portion. More specifically, the wire size is adjusted such that the wire is fused when a fusing current exceeding a predetermined current value flows therethrough. Thus, with the flow of the fusing current, the wire is fused before the semiconductor component of the power control device breaks down, thereby shutting down the current flow through the semiconductor component.
However, in case where the power control device is utilized as a switching device for the circuit, the circuit employing the fuse as the external element, like the first example of the prior art, results an increased scale and cost.
On the other hand, the following problems 1 to 5 exist with the second example of the prior art wherein the wire also serves as the fusion portion.
(1) The definition of a desired value of fusing current requires the selection of a suitable wire from those of various sizes based on the characteristics of each semiconductor component and hence, additional effort and cost must be paid. PA1 (2) A thinned long wire suffers a significant voltage drop produced therein. Hence, it is uneconomical to employ such a long, thin wire for forming the power control device for use in the circuit. PA1 (3) Difference in the pressure applied during the wire bonding results in deformation of wires which varies the predetermined fusing current and hence, the reliability of the power control device is decreased. PA1 (4) Heat generated from the fusion of the wire is instantaneously conducted to the semiconductor component via the wire, so that the semiconductor component may sometimes be expanded to rupture and hence, the semiconductor component becomes disabled to provide the normal operation. PA1 (5) As to a semiconductor chip having a large electrode and a wire of a large sectional area for use in large-current apparatuses, it is difficult to adjust the wire size to a desired value of fusing current. PA1 a pad portion connected to one end of a wire having another end connected to an external terminal; PA1 a contact portion contacting the semiconductor chip; and PA1 the fusion portion interconnecting the contact portion and the pad portion. PA1 the electrode including the fusion portion is composed of a pad portion connected to one end of a wire having another end connected to an external terminal; a contact portion contacting the semiconductor chip; and the fusion portion for interconnecting the contact portion and the pad portion, PA1 the contact portion, an electric insulating layer and the pad portion are sequentially laid in lamination on the semiconductor chip, and PA1 a through-hole defining the fusion portion is formed in the electric insulating layer.